The operation of microwave communications paths is regulated by governments, international bodies, and treaties. In the United States, use of microwave frequencies for communications is authorized for non-exclusive uncoordinated use and exclusive coordinated use. An example of uncoordinated use is Wireless Local Area Networks (WLANs) (e.g., operation in the Industrial Scientific and Medical (ISM) and Unlicensed National Information Infrastructure (UNII) bands operating under the Code of Federal Regulations (C.F.R.), Title 47, Part 15 (2002), often using IEEE 802.11 standards), where users are subject to interference from other users communicating on the same portion of the microwave spectrum. An example of exclusive coordinated use is fixed Point-To-Point (PTP) licensed systems (i.e., systems where the connection is strictly between two endpoints) operated for backhauling large data traffic between distributed sites and more centralized sites under 47 C.F.R.§101. For example, frequency bands between 3.7 GHz to 11.7 GHz (e.g., 3.7 GHz to 4.2 GHz, 5.9 GHz to 6.4 GHz, and 10.7 GHz to 11.7 GHz) can be used for fixed PTP licensed systems in the United States.
In the United States, an applicant for an exclusive fixed PTP license, for example from the Federal Communications Commission (FCC) or the National Telecommunications and Information Administration (NTIA), engineers the system, by appropriate studies, analyses, and selection of sites, transmitters, antennas, and frequencies to avoid interference in excess of permissible levels to other users in an Exclusive Use Area (EUA). Conventionally, fixed PTP systems on a coordinated microwave path operate in Frequency Division Duplex (FDD) mode, requiring two frequencies (i.e., one frequency for transmission in each direction between the two endpoint sites). For a fixed PTP path, the EUA boundary for the two frequencies is typically 125 miles from the antenna of a site, except for the antenna main lobe (about ±5 degrees), where the distance to the EUA boundary is typically 250 miles.
Generally, the prospective user of a proposed microwave path conducts both a site survey and a coordination study. The site survey includes verifying that there is a Line-Of-Site (LOS) path between the proposed endpoint sites and that the proposed sites can support the antennas. For example, the antenna support structure typically needs to withstand winds up to 125 mph. A coordination study is an analysis to identify any potential interference issues between the proposed sites and existing sites in the proposed EUA.
The proposed path applicant needs to conduct a prior coordination study, in accordance with government regulations, to ensure that the Equivalent Isotropic Radiated Power (EIRP) from the proposed path will not cause harmful interference (i.e., interference above a particular level) to other users operating on existing coordinated microwave paths on the proposed frequency pair and adjacent frequencies in the proposed path's EUA. A coordinated microwave path is a microwave path that has undergone a coordination study, as will be described in more detail below. An operator of a coordinated microwave path can receive a license to operate by a regulatory agency, e.g., the FCC. Once a proposed path is successfully prior coordinated and deployed, thereby becoming an existing path, it is the responsibility of the next proposed path applicant to ensure that a new proposed path does not cause harmful interference to the existing paths in the EUA.
If a proposed path applicant is unable, after conducting a prior coordination study, to prior coordinate a microwave path in an EUA for a particular frequency pair, the applicant may seek to prior coordinate the proposed path on a different frequency pair. However, this may not be possible, because the amount of spectrum in any given geographical area is finite. If it is possible, the path applicant performs a second, costly and time-consuming coordination study, issue a Prior Coordination Notice to all operators within the EUA, wait thirty days for any comments, and, if no adverse comments are received, wait for approval after regulatory filings. Additionally, if the proposed path applicant seeks to use an existing coordinated path site as one of the sites for the proposed path, an additional antenna and transceiver would be needed at the existing site to operate on the new frequency pair.
A conventional way to increase spectral efficiency is to configure a transmitter antenna to minimize the area in which harmful interference is produced. This may require minimizing the angle of the antenna radiation pattern main lobe and maximizing suppression of the antenna radiation pattern side lobes and back lobe. Typically, achieving narrow beam widths requires physically large, unsightly antennas, which can be expensive and can require substantial mounting structures to withstand high wind loading.